This invention relates to peak flow meters for monitoring the peak flow expiratory rate of a subject.
In the treatment of such conditions as asthma it has been recognised that regular sampling of peak flow can reveal a trend in the state of the condition and provide a useful warning to the subject to take action that will prevent the condition worsening. Typically, the physician will assess the subject to establish a normal peak flow expiratory rate and at the same time set lower warning limits. For example, if the performance of the subject falls below a first lower limit set, that indicates that a change of medication is needed, and the subject is instructed to seek immediate medical attention if it falls below a still lower limit.
The normal and warning limit values set will be specific to each subject and can be indicated on the body of the peak flow meter in a number of ways. Thus, it is known to provide markers in the form of self-adhesive stickers of different colours which are placed on the body of the instrument adjacent the reading scale at the appropriate scale values. It is also known to employ an adhesive strip (GB 2247838) which the physician can adjust to show different colour zones over particular peak flow ranges. In another alternative (U.S. Pat. No. 5,246,010), markers in the form of small friction pads are held captive in a channel on the body of the meter and are slid to the appropriate positions along the length of the reading scale, the frictional engagement of the pads deliberately requiring considerable force to displace the pads in order that they will not slide accidentally out of position.
These known forms of marker each have their disadvantages, however. Adhesive elements or strips can peel away in time, especially as the body of the instrument will continually be handled by the subject. The use of friction pads will not prevent the set values being disturbed by rough handling or by tampering, which is likely to occur if only out of curiosity, especially if the subject is a child. If the frictional force is increased to the extent that a child cannot shift the pads, it may become difficult for the physician to set them precisely to the values required.